1. Field of the Invention
This invention pertains to the general field of abrasive substances used to coat the cutting surfaces of metal working tools in order to improve their performance and longevity. In particular, it provides a new and improved way of applying a thin layer of titanium nitride coating over cubic boron nitride plated grinding wheels.
2. Description of the Prior Art
Abrasive substances are used for cutting, smoothing and polishing the surface of other hard materials. Naturally, in each instance the abrasive used must be harder than the surface it is working on in order to ensure proper performance and durability. Therefore, materials known for their high degree of hardness, such as diamond, aluminum oxide and silicon carbide, have long been used as the cutting edge of tools to improve their quality in grinding, sawing, lapping, machining and drilling. The resulting tools reduce production time and costs, and facilitate the maintenance of consistently high tolerances with little or no effect on the metallurgical integrity of the final product. Typically, the working surface of a tool made of steel or other metallic substrate is coated with a layer of harder and more abrasion-resistant material by chemical bonding, electrometallurgical deposition, or other methods of application. Thus, in addition to the properties of the abrasive material itself, the performance of the tool is affected by the quality and durability of the bonding between the abrasive particles and the tool substrate. U.S. Pat. No(s). 3,464,804, 3,645,706, 3,650,714, 3,859,057, 3,879,901 and 3,929,432 describe several processes and products directed at improving both of these critical characteristics in grinding tools.
Because of its hardness, for a long time diamond was the only abrasive capable of cutting very hard superalloys and composite materials, but it could not be used effectively on steels and other ferrous surfaces because of its tendency to react and be absorbed into such surfaces at the temperature and pressure conditions normally occurring in the grinding and machining process. A great improvement was achieved by the development of the synthetic superabrasive substance cubic boron nitride (CBN), marketed by the General Electric Company under the trademark "Borason," which exhibits abrasive properties comparable to diamond and can be used effectively for grinding hardened steels. In fact, the performance of CBN is far superior to that of aluminum oxide, the conventional abrasive used on hardened steel tools.
The manufacture of most CBN grinding wheels currently found in the industry is based on four major types of bonding techniques to incorporate the CBN cutting surface into the wheel's metal core, which generally consists of steel. These techniques include resin bonding, metal bonding, vitreous bonding and electroplating. The resulting CBN-coated grinding wheels are now routinely used (generally referred to as Borozon wheels) in the machining of hard ferrous metals, cast irons, and nickel-base and cobalt-base superalloys. They can produce more goods, to a higher degree of accuracy and at a lower cost than any other abrasive. In fact, the limiting factor in the life of such tools is typically determined not by wear on the cutting surface but by its break-down and separation from the metal core resulting from failure of the bonding layer.
Therefore, any process, material, or technique that might result in the lengthening of the life of Borozon wheels would be of great usefulness and commercial value to the industry. The present invention deals with a new of approach to strengthening the bond between the metal core (substrate) of the grinding wheel and the CBN layer on its cutting surface. The approach is based on the physical vapor deposition of titanium nitride on the CBN-bonded surface.
Nitrides and carbides have been used for some time in thin coatings to provide wear protection to moving parts of equipment and machinery. This protection has become particularly critical n recent years in automated production processes, where machine break-downs caused by wear of individual parts are a significant component of overall costs. Therefore, thin layers of these materials (specially titanium nitride) have proven very successful in reducing operating costs. They are extremely hard, abrasion-resistant, and adhere well to the supporting surface even in very thin layers, which is very important for the dimensional tolerances required for the coated part.
In general, the deposition of these thin layers of coating materials has been achieved at relatively high temperatures by chemical vapor deposition and by physical vapor deposition. Typically, though, titanium nitride is deposited at temperatures in the order of 1,200 to 1,700.degree. F., which, when applied to Borozon wheels, tends to affect the integrity of the substrate and the stability of the CBN-steel bond. This invention is based on the discovery that the physical vapor deposition process can be used successfully at relatively low temperatures to produce titanium nitride coated Borozon wheels of improved durability and efficiency.